1. Field of the Invention
The present invention relates to a developing device, an image forming apparatus and an image forming method. In particular, the invention relates to a developing device for developing a nonmagnetic monocomponent toner in a noncontact system, an image forming apparatus including the device, and an image forming method using the device.
2. Description of the Related Art
In image forming methods of electrophotography system such as copying machines and printers, electrophotographic photoconductors (photoconductor drums) have been widely used as latent image carriers. A general image forming method using such an electrophotographic photoconductor is practiced as follows.
A surface of an electrophotographic photoconductor is charged to a predetermined potential using charging means, and light from an LED light source is applied to the surface using exposure means. The potential in the exposed area is thereby optically attenuated to form an electrostatic latent image corresponding to an image. Subsequently, by applying a developing bias to the developing device including a toner carrier, a toner is moved onto the electrophotographic photoconductor and the electrostatic latent image is developed therewith to form a toner image on the surface of the electrophotographic photoconductor. Finally, the toner image formed is transferred to an intermediate transfer body or paper by bringing the electrophotographic photoconductor into contact with or close to transfer means.
The above-mentioned electrophotographic photoconductors can be classified roughly into inorganic photoconductors whose photo sensitive layers each are composed of an inorganic material, such as amorphous silicon, and organic photoconductors whose photo sensitive layers each are composed of an organic material.
Among these, inorganic photoconductors, especially amorphous silicon photoconductors, are widely used because they have so high mechanical strength that their photoconductive layer is resistant to wear even in repeated use and therefore they have an advantage that good quality images can be supplied stably.
However, such amorphous silicon photoconductors have higher dielectric constants in their photoconductive layer in comparison to organic photoconductors. Therefore, some problems have been recognized; for example, a toner tends to adhere to the surface of a photoconductive layer firmly and, as a result, image density unevenness easily occurs due to insufficient detachment of a toner during a developing step.
In particular, when developing a nonmagnetic monocomponent toner in a noncontact system, a toner is caused to fly by a developing bias applied between a toner carrier and an electrophotographic photoconductor and thereby the toner is moved from the toner carrier to the electrophotographic photoconductor. In this event, an attempt to inhibit the adhesion of a toner to the surface of a photoconductive layer tends to cause a problem of insufficient flying of the toner and it has been very difficult to find suitable developing conditions.
In order to prevent a toner from adhering firmly to the surface of a photoconductive layer, a method of effecting development by applying an AC bias to a toner carrier has been disclosed in JP 2003-122047A while not being limited particularly to amorphous silicon photoconductors.
More specifically, JP 2003-122047A discloses an image forming method in which the AC bias is limited to a peak-to-peak electric field strength of from 3×106 to 1×107 V/m and a frequency of from 100 to 5000 Hz.
Moreover, in JP 2003-122047A, a gap between an electrophotographic photoconductor and a toner carrier is set within the range of from 100 to 500 μm, and a speed ratio of the electrophotographic photoconductor and the toner carrier is specified within the range of from 1.02 to 3.0.
In the image forming method of JP 2003-122047A, however, no consideration is made to the difference of dielectric constant between electrophotographic photoconductors used. Moreover, no clear distinguishment about the type of toner, namely, magnetic or nonmagnetic, monocomponent or dicomponent, is made about the toner to be used. It may be said that the image forming method is a very rough method.
Therefore, it has been difficult to optimize developing conditions when using an amorphous silicon photoconductor as an electrophotographic photoconductor and a nonmagnetic monocomponent toner as a toner, and practicing the development in a non-contact system. In other words, there has been a problem that it becomes difficult to control an adhesion force of a toner to an amorphous silicon photoconductor and it is impossible to inhibit the occurrence of image density unevenness effectively.